Mike, your post made me really want to know whether or not I should have, besides a GFCI, a ground probe in my aquarium.
I finally found something that convinced me I do want a probe
http://www.youtube.com/watch?v=zpBUBzk2ZIU
Imagine sticking your hand into your aquarium and you create the ground through some other piece of equipment such as a fixture, or a wet floor, or whatever.
Sure the GFCI will trip, but you will still recieve a jolt.
If your heart is healthy enough, I guess it wouldn't bother you
